1. Field of the Invention
The present invention relates to a micromirror actuator, and more particularly, to a micromirror actuator which is capable of effectively preventing a mirror from abnormally operating due to abnormal variation of a torsion bar.
2. Description of the Related Art
Micromirror actuators are optical switching devices used in optical communication devices and holographic optical information recorders. In holographic optical information recorders, mirrors are required to be placed at exact positions for correctly changing an optical path, that is, for switching. A plurality of micromirror actuators are installed in an array in a holographic optical information recorder and must show the same mirror operational characteristics.
FIG. 1 illustrates a conventional micromirror actuator, in which two posts 2a and 2b are installed a predetermined distance apart on a substrate 1, and torsion bars 3a and 3b are formed to extend from a mirror 3. The torsion bars 3a and 3b are connected at one end to the two posts 2a and 2b, respectively. In addition, magnets 4a and 4b are placed at two opposite sides of the mirror 3.
Referring to FIG. 1, the mirror 3 inclines over the substrate 1 by a predetermined angle due to a vertical magnetic field emanating from the substrate 1. Here, one edge of the mirror 3 is in contact with the surface of the substrate 1, and thus the inclination angle of the mirror 3 with respect to the substrate 1 can be maintained. When the mirror inclines over the substrate 1, the torsion bars 3a and 3b having predetermined elastic forces are twisted.
FIG. 2 illustrates the conventional micromirror actuator of FIG. 1 in a state where there is no magnetic field. If the magnetic field affecting the micromirror actuator of FIG. 1 is removed, as illustrated in FIG. 2, the mirror 3 rotates about the torsion bars 3a and 3b so as to be parallel with the surface of the substrate 1 due to the elastic restoring force of each of the torsion bars 3a and 3b. 
As shown in FIG. 1, when the mirror 3 is inclined over the substrate 1 by 45 degrees, light, which enters the micromirror actuator of FIG. 1 parallel to the surface of the substrate 1, is reflected perpendicular to the substrate 1 by the mirror 3. On the other hand, light, which enters the micromirror actuator of FIG. 2, directly passes over the surface of the mirror 3 without being reflected by the mirror 3. The operation of the micromirror actuator is controlled by the external magnetic field mentioned above. In most cases, an electromagnet is attached on the bottom surface of the substrate 1 in order to form such a vertical magnetic field.
As shown in FIG. 3, when an external magnetic field is formed, the mirror 3 can rotate about the torsion bars 3a and 3b against the elastic forces of the torsion bars 3a and 3b so as to form a predetermined angle with the substrate 1. On the other hand, when there is no external magnetic field, the mirror 3 rotates so as to be parallel with the surface of the substrate 1 due to the elastic restoring forces of the torsion bars 3a and 3b. 
However, as shown in FIG. 4, if a strong external magnetic field is applied to the micromirror actuator after one edge of the mirror 3 contacts the surface of the substrate 1, the torsion bars 3a and 3b cannot maintain their straight shapes and are inevitably bent due to their flexibility. Here, the fact that the torsion bars 3a and 3b cannot maintain their straight shapes, means that the rotation center of the mirror 3 changes and accordingly, the rotation angle of the mirror 3 exceeds a desired level. The torsion bars 3a and 3b are supported at one end by the posts 2a and 2b, respectively. Accordingly, it becomes difficult to obtain a normal inclination angle of the mirror 3 in the conventional actuator, in which the mirror 3 and the torsion bars 3a and 3b are connected to one another. Dotted lines 3axe2x80x2 and 3bxe2x80x2 in FIG. 4 indicate the original shapes of the torsion bars 3a and 3b, respectively, before the lower edge of the mirror 3 contacts the substrate 1 and the torsion bars are abnormally deformed. Solid lines in FIG. 4 indicate the shapes of the torsion bars 3a and 3b, respectively, abnormally deformed due to the rotation of the mirror 3.
As described above, if the mirror 3 is sufficiently rotated until one edge of the mirror contacts the surface of the substrate 1 and thus the torsion bars 3a and 3b are deformed, the rotation center of the mirror changes, and the rotation angle of the mirror 3 exceeds a designed angle range. Accordingly, it is impossible to reflect light in a desired direction in an apparatus using the conventional micromirror actuator as an optical switching device.
To solve the above problems, it is an object of the present invention to provide a micromirror actuator which is capable of precisely controlling the rotation angle of a mirror and preventing light from being reflected into an abnormal path due to abnormal rotation of the mirror.
Accordingly, to achieve the above object, there is provided a micromirror actuator including a substrate; two posts having predetermined heights and installed a predetermined distance apart on the substrate; a element, such as a torsion bar both ends of which are fixed to the posts; a stopper extending from a portion of the torsion bar and contacting or coming apart from the surface of the substrate, e.g., contacting or not contacting a surface of the substrate depending on a state of the torsion bar; a mirror connected to a portion of the torsion bar such as its middle; a driving element, such as parallel elements, connected to the torsion bar being isolated from the mirror and causing the torsion bar to be distorted; and a magnetic component, such as a magnet providing a rotation force to the driving element via an external magnetic field.
Preferably, the mirror is connected to the middle portion of the torsion bar via a connecting portion. Preferably, the parallel elements are symmetrically installed at two opposite sides of the mirror and the magnet is installed on each of the parallel elements. Preferably, the parallel elements are connected to each other via a connecting element and thus are kept in alignment with each other, and the magnet is formed on at least one of the parallel elements.
Preferably, the stopper is formed at the middle portion of the torsion bar and is opposite to a connecting portion connecting the mirror and the torsion bar.